Help with the HD attached to my Apple Airport Express to show up on my Playstation 3.

Hi, I have the Apple Airport Extreme. I have connected it to my ISP router and attached a WD 2TB HardDrive. I can access the HD wirelessly via my MacBook, PC, Mobile & Tablet. Now, my PS3 is also connected to the same network wirelessly. On the home screen (of the PS3) where it says Pictures, Music, Video etc - the HD doesnt show up. It picks up my sisters laptop (windows 7) and shows her laptop as a media server, but the HD connected to the AAE (Apple Airport Express) doesnt show up. Even after I pressed "Search for Media Servers". Hmmm.. help? Thanks!